The piloting of an aircraft or helicopter is recognized as an extremely complex task because the aircraft or helicopter moves with six degrees of freedom, three degrees of translational freedom and three degrees of rotational freedom. To aid in this flying, projection displays were developed to display images, usually of the aircraft's instrument data, superposed against the scene of the real world (the environment). The first generation projection displays, also known as heads-up displays, consisted of a cathode ray tube with the appropriate optical elements mounted in the aircraft's instrument panel. The cathode ray tube generated an image which was reflected by a beam-splitter onto a combining mirror and then to the eyes of the pilot. Furthermore, through the appropriate selection of optical elements between the cathode ray tube and the combining mirror, the projected images on the combining mirror could be made to appear as if projected to optical infinity. This allowed the pilot to observe simultaneously both the aircraft's instrumentation and the real world scene.
In spite of the many obvious advantages, the first generation of projection displays had two major problems. First, the displayed information was stationary around an axis and was usually aligned along the longitudinal axis of the aircraft. Second, in any future aircraft design, large amounts of scarce instrumental panel space would be required for the large and bulky cathode ray tubes.
In an attempt to overcome the above mentioned problems, a second generation of projection displays, known as helmet mounted display, was developed. These second generation projection displays had the cathode ray tubes, the optics, and a high voltage cable mounted in the helmet. See U.S. Pat. No. 3,923,370, column 4, line 58 to column 5, line 42. The high voltage cable supplied the necessary voltage for the operation of the cathode ray tubes, but also had the potential for sparking during emergency disconnects. The helmet mounted cathode ray tubes were also very small in size, operated at voltages that were less than optimal, and produced generally dim images with poor resolution to the display optics. The optics in the second generation helmet mounted displays were also changed drastically because the cathode ray tubes were moved into the helmet. The images were produced at various locations in the helmet and projected, through a series of optical steps, down to the eyes of the pilot. The cathode ray tubes projected an image onto a partially reflective mirror, then onto a totally reflective mirror, and then to a combining mirror mounted on the visor in the pilot's line of sight. See U.S. Pat. No. 3,923,370, column 5, lines 1-42; U.S. Pat. No. Re 28,847, column 3, lines 58-65, column 4, line 52 to column 5, line 3. Hence, the pilot would see both the instrumentation and the real world scene simultaneously.
While the second generation of projection displays solve the problems discussed in connection with the first generation projection displays, it did suffer from a number of its own significant problems mainly associated with safety. First, during emergency ejection situations where both smoke and fuel vapors were in the cockpit, the high voltage cable had a tendency to spark. Second, this design forced the pilot to carry a great deal of weight on his head which restricted head movement, contributed to fatigue, and, during evasive maneuvers, could cause neck injury. Third, the cathode ray tubes within the helmet produced severe heat buildup within the helmet which resulted in degraded pilot performance. Finally, the cathode ray tubes mounted within the helmet operated at a lower voltage for reliability purposes, but this also produced dimmer images.
In an attempt to provide a more efficient projection display design, a third generation of projection displays was developed in which the cathode ray tubes were removed from the helmet, removed from the instrument panel, and placed in a noncritical portion of the aircraft with an optical fiber bundle coupling the cathode ray tubes with the eyes of the pilot. See U.S. Pat. No. 4,439,755, line 3, lines 1-5. With the removal of the cathode ray tubes from the helmet, the high voltage cable to the helmet was also removed and replaced with an optical fiber bundle which went to the optics within the helmet. The optics within the helmet were also changed. The images were produced by the cathode ray tube and were transferred from the optical fiber bundle through two compound lenses and a mirror. The mirror reflected and focused the images onto a fiber optical bundle in the helmet which carried the images to the front of the helmet and caused the images to be incident on a second mirror. The second mirror reflected the images onto a collimating lens and then to a beam-splitter which allowed the pilot to view both the image at infinity and the real world scene. See U.S. Pat. No. 4,439,755, column 7, lines 40-58, column 8, line 64 to column 9, line 11.
Although a rather good design, severe problems plagued the third generation of helmet mounted displays. First, the long fiber optical cable running from the helmet to the cathode ray tubes produced an unacceptable loss of light intensity and resolution between the cable's input and output. Second, since the distance between the helmet and the cathode ray tube was rather long, the fiber optical cable was rather thick and inflexible. Finally, the fiber optical cable required repair or replacement after every emergency disconnect situation because dust would inevitably enter into the fiber optic junction and cause severe distortion of the images seen by the pilot.
To further trace the development of projection optics see U.S. Pat. Nos. 3,291,906; 3,787,109; 4,220,400; and 4,508,424.